1. Field of the Invention
This invention relates generally to color organic light-emitting diode (OLED) displays, and more particularly, to improving the efficiency of use of white light emitted from the diode in producing a color display by incorporating a color conversion layer between the white light emitting diode and the color filter layer. The color conversion layer consists of nanocrystals, which are dispersed in a transparent binding material and may be employed in either down-emitting or up-emitting color OLED display devices.
2. Description of the Related Art
A light-emitting diode (LED) is a form of solid-state light source having a structure of a p-n junction. In an OLED, one or more layers of semiconducting organic material are sandwiched between two electrodes. An electric current is applied to the device, causing negatively charged electrons to move into the organic material(s) from the cathode. Positive charges, typically referred to as holes, move in from the anode. The positive and negative charges meet in the center layers (i.e. organic material), recombine, and produce photons. The wavelength of the photons, and consequently the color of the emitted light, depends on the electronic properties of the organic material in which the photons are generated.
The color of light emitted from the OLED display device can be controlled by the choice of the organic material. The color of light emitted by a particular structure can be controlled both by selection of the organic material as well as by selection of luminescent impurities, or dopants, added to the organic materials.
The color of the light emitted from the OLED display device can also be affected by color conversion layers and color filters placed in the path of the emitted light.
A color display requires the ability to provide a wide gamut of colors. In displays, this is accomplished by mixing various amounts of the primary colors: red, green, and blue in each of the individual picture elements, pixels. Thus, each pixel consists of subpixels, which define the color characteristics of the primaries used to compose the desired color. The amount of each primary color needed is dependent on both the distribution of wavelengths emitted by the OLED and the color-altering characteristics of the individual subpixels.
The color definition of the subpixels can be accomplished by one of two basic techniques. One approach provides a self-emissive pixelated display with red, green, and blue (RGB) subpixels placed next to each other. This approach, in principle, allows the best possible performance because no light is lost through filter absorption or color conversion. It requires, however, precise shadow mask fabrication and alignment in the process of vacuum deposition of the organic material(s). Such precision has become increasingly difficult as the resolution desired for the display increases. The second approach involves depositing the necessary organic materials to yield a white light emission from the OLED. This white light is selectively converted to the desired color by the use of a color filter patterned over the array of subpixels. The patterning of the color filter materials is readily accomplished with standard lithographic techniques. The major drawback of this approach is that much of the OLED emission is lost in the color filtering process.
A recent variation on the latter technique is disclosed in U.S. Pat. No. 6,608,439 to Sokolik et al., which limits the emission of the OLED to relatively short, high-energy wavelengths in the blue or near ultraviolet region of the electromagnetic spectrum. This high-energy emission is then down converted to less energetic green and red by color conversion. If a blue emission is used, then either no conversion or only minor filtering is necessary for the blue component. This color conversion can be accomplished either by use of organic materials which absorb the high-energy emission and photo-luminance or by inorganic nanocrystals. The organic materials have the characteristic of broad luminescent spectra, which results in the need for further color filtration and loss of emission. The inorganic nanocrystals have recently gained favor because of their narrow emission bands and the ability to tune the wavelength characteristics by controlling the size of the particles.
Although this high-energy variation is attractive in principle, presently, the availability of the necessary materials is lacking. Present materials do not have enough emission intensity and lifetime to make a display that is commercially competitive. As such, while this prior art may be suitable for the particular purpose employed, or for general use, it would not be as suitable for the purposes of the present invention as disclosed hereafter.
The present invention improves on the prior art by taking existing white light emitting OLED display devices and increasing the efficiency with which they convert the light into the needed colors.
It is, therefore, a primary object of the present invention to increase the efficiency of use of white light emitted from a diode by incorporating a color conversion layer between the white light emitting diode and the color filter layer.
It is another object of the present invention to provide a color conversion layer that may be employed in either down-emitting or up-emitting color OLED display devices.
It is another object of the present invention to provide an increase in the red and green color emission output from a white light-emitting diode.
It is another object of the present invention to provide a display device with improved luminance.